There should be an option (in Settings) to add a background, and if it contains more than 1 frame and is GIF format, the game will automatically animate it. If the computer/mobile starts using too much memory, or the mobile/computer starts running low on battery/or heating up, then this is turned off. The animation can be automated (game detects number of frames and asks you what speed to set it on ranging from 0.5x to 5x), or manual, and can be turned on/off in the settings. Plus mobile devices cannot access the Synthesia Configuration Tool.

I've been thinking about making the background image option (in the video creator, at least) accept video files as input. It might take a bit of work to synchronize the background with the song, but that would be neat.

I like the idea of it accepted video, but another nice option might be the option to select from a list of "visualizers", something that animates with the song, then there wouldn't be a syncing issue. It would also be awesome if this used a plug in framework, like what Windows Media Player did back in the day, so add and use their own animations. This is dated, but demonstrates the visualizers: https://www.youtube.com/watch?v=Xq_7qZrSi4A